<!DOCTYPE html>
<html lang="zh_CN">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .first{
            width: 500px;
            height: 500px;
            background-color: blue;

            border: 10px yellow solid;

            padding: 10px 20px 30px 40px;
            /* 上右下左 顺时针方向 */
            /* 盒子的可见大小由内容，内边距，边框所决定 */
        }
        .second{
            width: 100%;
            height: 100%;
            background-color:blueviolet;
        }
    </style>
</head>
<body>
    <div class="first">
        <div class="second"></div>
    </div>
</body>
</html>